Futurize for python2/3

Context

This merge request brings compatibility for python 3 while ensuring backwards compatibility with python 2.

Details

The future package has been used to futurize the code. Waves can now depend on python3-xxx instead of python-xxx packages.

If python 2 is to be used instead of python 3, proceed with the usual build and add -DUSE_PY3=OFF to the cmake command line. Note that the additional python-futurize package is required.

Test

Passed on ubuntu18.04 (python 2.7.15 and 3.6.9), debian (gaston, python 2.7), msys2 (python 2.7.16 and 3.8.1)

CMakeLists.txt

@@ -99,8 +99,8 @@ IF(USE_PY3)
 ELSE()
     SET(pyVersion 2.7)
 ENDIF()
-FIND_PACKAGE(PythonInterp ${pyVersion})
-FIND_PACKAGE(PythonLibs ${pyVersion})
+FIND_PACKAGE(PythonInterp ${pyVersion} REQUIRED)
+FIND_PACKAGE(PythonLibs ${pyVersion} REQUIRED)
 
 MESSAGE(STATUS "PYTHON_EXECUTABLE:FILEPATH=${PYTHON_EXECUTABLE}")
 MESSAGE(STATUS "PYTHON_LIBRARY:FILEPATH=${PYTHON_LIBRARY}")